Messenger RNAs from the anterior and neurointermediate lobes of pituitaries of Xenopus laevis were used to construct cDNA libraries in the Pst I site of pBR322 cloned in strain MC 1061 of E. coli K12. One of these cloned plasmids contains a 435 bp insert which hybridizes to a probe of mouse pro-opiomelanocortin (POMC) cDNA. This probe also hybridizes on southern blots with portions of each of 3 different clones of Xenopus genomic DNA in phage Lambda. We are presently sequencing the 435 bp insert and attempting to identify POMC-containing restriction fragments from the Lambda clones for future subcloning and sequencing. We have also developed libraries of 400,000 - 600,000 clones of cDNAs for sequences expressed in differentiated cells of NS20Y mouse neuroblastoma cells and the neuron-glia hybrid cell line NG108cc15. We are presently selecting probes of differentiation-specific sequences from each of these cell lines by cascade hybridizations with heterologous (undifferentiated cells of the same cell line) and homologous mRNAs. A library of several million clones of high molecular weight cDNAs from NG108cc15 cells has been prepared in the expression vector Lambdagtll. Portions of this library are being screened with monoclonal antibodies which have been prepared against the enzyme choline acetyltransferase from rat brain.